Accurate control of output power of a wireless system (e.g., a user equipment (UE), a base station such as an Evolved Node B (eNB), etc.) is important for several reasons. In frequency division duplexing (FDD) systems, the power control ensures accurate power step sizes and proper setting of the maximum output power. If the actual output power gets higher than the maximum linear output power capability of the wireless system, then linearity performance metrics such as the adjacent channel leakage ratio (ACLR) and error vector magnitude (EVM) will degrade. The accurate control of the maximum linear output power is one of the key tasks of a power control scheme.
FDD systems have tight requirements when changing the output power. In such systems, the relative power tolerance is more important than the absolute tolerance. For example, the Third Generation Partnership Project (3GPP) Technical Specification (TS) 25.101 requires, for 3G FDD systems, a maximum error of ±0.5 dB for every 1 dB change in output power.
For time division duplexing (TDD) systems such as the Global System for Mobile Communications (GSM) or Long Term Evolution (LTE) TDD implementations, the absolute power tolerance is more important than for FDD systems. TDD systems feature discontinuous transmission normally implemented as a slot-wise transmission. At the beginning of each transmit slot, the output power ramps up to the desired target value during the transmit slot. After each transmit slot, the power ramps down to a specified minimum level. In LTE TDD systems, the challenge is to ramp-up from a low output power (e.g., <−40 dBm for LTE TDD) to up to 23 dBm, depending on the target power for the transmit slot, while keeping the output power tolerances and without exceeding the maximum output power capability of the wireless system.
Thus, power control is an important task for wireless systems, and remains just as important in systems with simultaneous transmission in two or more frequency ranges.